Among the various ultrasonic imaging techniques, one technique provides focusing of at least the received beam, but typically both the transmitted beam and the received beam, to a scan area, point or line which extends through a certain depth in the body under examination, from the transmission surface of the transducers into the body under examination.
During beam transmission, the acoustic energy generated by the transducers are concentrated upon the particular area, line or point. During beam reception, beamforming techniques are used to receive and synchronize the components of the signals received by the receiving transducers, and reflected by the particular area, line and point.
The time delays imparted to the reflected signals are a function of the propagation velocity of acoustic waves in the body under examination, of the steering angle of each transducer relative to the reflection line or point, and of the position of each receiving transducer in its array. Transducer arrays of ultrasonic probes may be of different types, provided that transducers are arranged side-by-side over at least one straight or curved line, or over a flat or curved surface.
In other types of probes, a transducer array having a smaller number of transducer is translated or rotated to cover a wider field of view. With a few obvious variants, the above method also applies to these types of transducer arrays, by simply accounting for their movement during scanning.
In the latest generations of ultrasonic imaging apparatus, especially those used in combination with contrast agents, imaging is based on the part of the received signal which frequency is a harmonic of the fundamental transmission frequency. In this case, the fundamental frequency component of the received signal must be separated and/or suppressed to allow imaging. This is mainly due to the fact that the fundamental frequency component of the signal has a much higher amplitude than harmonic frequency components, such that it generates imaging noise.
In prior art, separation of the second harmonic component of the received signal from the fundamental transmission frequency component is obtained by using various techniques. One of these techniques provides the use of adaptive filters which require a greater computational load in the received signal processing process, and also require a certain knowledge of the received signal.
Other techniques for separating the second harmonic component of the received signal from the fundamental frequency component use a double firing of acoustic pulses wherein the corresponding received signals are combined in such a manner as to cause the suppression of the fundamental frequency component.
One of these techniques is, for instance, the so-called “pulse inversion” technique, in which two transmit pulses are phase inverted and their received signals are summed. These techniques avoid the use adaptive filters, but require double imaging times as each scan line require the transmission of an additional ultrasonic pulse.
U.S. Pat. No. 6,193,663 discloses a medial diagnostic ultrasonic imaging system which acquires received beams from spatially distinct transmitted beams. The transmitted beams alternate in type between at least first and second types across the region being imaged. The first and second types of received beams differ in at least one scan parameter other than transmission and receiver line geometry and can, for example, differ in transmission pulse phase, transmission focus, transmission or receiver aperture, system frequency complex phase angle, transmission code or transmission gain. Received beams associated with spatially distinct ones of the transmitted beams (including at least one beam of the first type and at least one beam of the second type) are then combined. In this way, many two-pulse techniques can be used while substantially reducing the frame rate penalty normally associated with such techniques. The transmitted beams are transmitted at different times. The delays used for forming the received beams depend only from the direction, i.e. the so called steering of the beam and from the position of the transducers forming the probe.
U.S. Pat. No. 6,186,950 also discloses an ultrasound imaging apparatus combining the received beams generated from different transmitted beams transmitted at different successive instants.
U.S. Pat. No. 6,193,663 and U.S. Pat. No. 6,186,950 do not modify the classical techniques of beamforming, either for the transmitted beams and for the received beams.
The invention has the object of providing a method like the one described hereinbefore, which allows to separate the fundamental frequency component of the receive signal from the component at one of the harmonics, especially the second harmonic, of said fundamental frequency, by using a method which is as simple as the methods that allow such separation by combining the receive signal of two successive identical transmit pulses without requiring said double transmission, thereby reducing imaging or scanning times.